This invention relates to the production of nickel hydroxide in general, and more particularly, to a process for removing the by-product sodium sulfate from the resulting effluent and recycling the sodium sulfate depleted solution back to the nickel hydroxide production process.
Nickel hydroxide (Ni(OH)2xe2x80x94also known as nickelous hydroxide and divalent nickel hydroxide) is an essential compound in alkaline cells, metal hydride batteries and other industrial and commercial applications. Moreover, nickel hydroxide is a precursor of nickel oxidexe2x80x94a critical industrial chemical having a myriad of uses.
Most commercial processes for making nickel hydroxide rely on its caustic precipitation from nickel salt solutions (nickel sulfate, nickel chloride or nickel nitrate) containing ammonia/ammonium salts.
Assignee has developed alternative methods for directly producing nickel hydroxide by utilizing elemental nickel powders. See U.S. Pat. No. 5,447,707 to Babjak et al., U.S. Pat. No. 5,824,283 to Babjak et al. and U.S. Pat. No. 5,545,392 to Babjak et al. However, most commercial nickel hydroxide producers still employ variations of the traditional caustic precipitation technique.
Accordingly, precipitation of chemical compounds from sulfate solutions using a sodium base (sodium hydroxide or sodium carbonate) generate prodigious amounts of sodium sulfate as a by-product. In conventional nickel hydroxide production, the precipitation is usually carried out from an approximate a 2 M nickel sulfate solution, containing ammonia (NH3), in a strong sodium hydroxide solution according to the overall reaction:
NiSO4(aq)+2NaOH(aq)xe2x86x92Ni(OH)2(solid)+Na2SO4(aq) 
One mole of sodium sulfate by-product is generated per each mole of nickel hydroxide product. Large quantities of effluent typically contain about one mole/liter of sodium sulfate, about 0.5 mole/liter of ammonia and small quantities of nickel and possibly other elements. Discharge of this effluent is environmentally unacceptable.
Many regional U.S. and Canadian environmental regulations call for the following limits:
xe2x89xa6100 mg/L of Kjeldahl nitrogen (corresponding to xe2x89xa6121 mg/L of ammonia provided no other nitrogen compounds are present in the effluent)
xe2x89xa63 mg/L of nickel
xe2x89xa61500 mg/L of sulfate
5.5xe2x89xa7pHxe2x89xa79.5
xe2x89xa665xc2x0 C. effluent temperature
The presence of free ammonia causes the nickel to complex with it thereby hampering the precipitation process. Nickel amines, for example NiNH3++, formed by difficult-to-break covalent bounds between the nickel and the hydrogen, create impediments to precipitation. The nickel tends to stay in solution. Diluting the effluent with water, in order to achieve the allowable limits, is against U.S. Environmental Protection Agency and other regulations. Although the sulfate specification might be less severe in some jurisdictions, the removal of ammonia to the allowable level is always necessary. This requires subjecting the entire effluent stream to an ammonia distillation step using a tall distillation column which is a rather costly operation. Moreover, the concentration of nickel must be reduced to the desired level and the pH of the solution must be adjusted before the effluent can be safely discharged. The added burden is time consuming, equipment intensive and costly.
A process for removing sodium sulfate from effluents by crystallization at relatively low temperatures. The sodium sulfate crystallizes as pure Na2SO4.10H2O compound (known as Glauber""s salt, mirabilite and sodium sulfate decahydrate). The mother liquor containing the ammonia and nickel ions may be recycled back to the original process.